For many years a pair of superimposed rotary dies with cutting blades on one or both of the dies have been used to cut blanks from a thin web of material passing through the nip of the dies. Typically, the thin web is of a material such as paper, paperboard, cardboard, plastic film, metal foil, thin sheet metal, and the like.
U.S. Pat. No. 4,608,895 discloses a pair of rotary dies which are made from a cylindrical blank of tool steel which is hardened and then ground to produce a cylindrical surface before it is electrical discharge machined (EDM) to produce cutting blades on the surface of the dies with the desired geometry to cut the blanks. The cutting blades are formed on the rotary dies after the steel of the rotary dies is hardened to prevent the deformation which would occur if the cutting blades were formed and then the rotary dies were heat treated to harden the cutting blades. While these rotary dies have been sold commercially and performed satisfactorily for relatively high volume mass production operations, they are expensive and difficult both to manufacture and re-sharpen when they become worn or dull in use.
U.S. Pat. No. 5,575,185 discloses a pair of rotary dies having coacting cutting blades projecting generally radially outwardly from the main body of each rotary die. These rotary dies are formed of unhardened tool steel and the cutting blades may be formed by machining with conventional cutting tools. After the cutting blades are formed by machining, the cutting blades are hardened by heating to an elevated temperature with a laser and quenching them without substantially hardening the core or body of the workpiece and hence without distorting the core or body of the workpiece by heat treating it. Thus, this system obviates the need for EDM machining which reduces the cost to produce rotary dies.
However, conventional cutting tools tend to produce an undesirable offset or undercut when attempting to machine a cutting blade which extends generally radially outwardly from a rotary die cylinder. The undercut produces a cutting blade with a sharp edge or edges which wear quickly and may chip causing an unsatisfactory cut. Further, the blades with an undercut can cause a cut blank of material to become stuck on a cylinder interfering with the cutting of subsequent blanks and potentially jamming and damaging the cutting dies. Also, detailed portions of the cutting blades are difficult to form with conventional cutting tools and on average, 30% to 60% of the total machining time of cutting blades with conventional tools is spent forming such detailed portions as corners and cross-over portions.